SECTIONAL TRANSACTIONS.— K. 387 



alpina is not its non-viability in such water, but because its behaviour 

 changes under these conditions. The animal becomes persistently nega- 

 tively rheotactic, and so is never able to colonise streams with an organic 

 content above a certain value. 



The presence of Planaria alpina may be taken as an indication that the 

 supply of water is not only constant, but also of very low organic content. 



Dr. L. Lloyd. — Polytoma as an indicator of oxygen deficiency. 



Previous applications of the aerotaxis of certain micro-organisms as indi- 

 cators of the oxygen tension in water have been by microscopic methods. 

 The reaction here described, being macroscopic, has a more practical interest. 

 The organism employed is obtained in culture in a medium heavier than 

 and not readily miscible with water. A sample of the culture is placed in a 

 narrow upright tube and the water to be tested is run on to this to a chosen 

 height. The organisms gather in blanket-like formation at the junction 

 of the fluids, and this mounts the tube at a speed which is influenced by the 

 oxygen tension. Spirillum is present in the culture employed and accom- 

 panies Polytoma in the blanket. Nitrates and nitrites in the water cause a 

 proportionate lag in the climb. Variations in the activity of the cultures 

 prevent as yet absolute readings being obtained, and the test is read in rela- 

 tion to controls of a standard water. The test is thought to be a possible 

 biological alternative to the Oxygen Absorption Tests as applied to con- 

 taminated waters, and is probably niore selective in its action than the 

 chemical reagents. Details of the technique are demonstrated and various 

 tests set up which show the delicacy of the reaction. A grading of waters 

 by means of the Polytoma reaction is contrasted with that obtained by their 

 chemical analysis. 



Afternoon. 



Prof. S. G. Paine. — Bacteria in relation to the decay of stone. 



During recent decades biology has been found to enter many fields 

 where its influence was least expected. In soil chemistry it has helped 

 to explain much of the phenomena which result in the conversion of rock 

 into fertile soil, and now we find our building stones, though for the most part 

 removed from conditions where bacterial action is possible, yet subject to 

 destruction by soil bacteria in much the same way as the native rock. At 

 the Glasgow meeting of the Association, R. M. Buchanan showed that 

 bacteria were associated with decaying stone ; Stutzer and Hartleb, in 1899, 

 suggested that nitrifying bacteria may contribute to the disintegration of 

 cement, and the nitrifying bacteria were believed by J. E. Marsh (1923) 

 to be responsible for a considerable part of the decay of college buildings 

 at Oxford. The present paper embodies the results of seven years' investi- 

 gation of the problem of stone decay under the aggis of the Building 

 Research Department of the Scientific and Industrial Research Board. 

 Common air and water organisms are nearly always present in decayed 

 stone in surprisingly large numbers. It has also been clearly shown that 

 organic matter present in rain-water is sufficient to allow of the development 

 of acid bacterial products which attack progressively the substance of the 

 stone. The presence of nitrifying bacteria has been established , and perhaps 

 most significant of all a new autotrophic bacterium capable of oxidising 

 sulphides, sulphites and thiosulphates has been discovered. Much of the 

 formation of the sulphate incrustations previously believed to be due to 

 sulphur dioxide of the atmosphere will probably be found to be due to the 

 action of these micro-organisms. 



